1. Related Inventions
This application is related to U.S. patent application Ser. No. 833,699, filed Feb. 27, 1986, which corresponds to European Patent Application Publication No. 193,911.
2. Field of the Invention
The present invention relates to a process for displaying information, especially on a flat panel display.
3. Background of the Invention
The present display technology is evolving rapidly towards flat panel structures replacing the bulky and power consuming CRT in a number of applications where high resolution is not a primary concern.
This new generation of displays covers two main categories, namely the active and passive displays.
Active displays generate light on a flat surface, according to the visual pattern to be displayed. They are very much like the screen of a CRT and present the advantages and restrictions of a CRT. For example, the visibility depends on the light environment and a bright light falling on the screen hampers its readability.
Passive displays selectively reflect or absorb light to form the pattern building up the image. They are similar to a printed sheet of paper. The higher the ambient light is, the better the picture. They are most suitable for operation in high ambient light conditions.
The addressing of a flat display is generally based on digital control over a crossbar conductor system where picture elements are located at orthogonal X-Y intersections. TV-type scanning is possible using shift registers for addressing the X-Y intersections row-by-row in raster fashion.
A number of flat displays have been suggested in the past decade, and the various types are distributed between the passive (light modulating) types such as: Liquid Crystals Displays (LCD); Magneto Optics Displays (MOD); and Electrochromic Displays (ECD); and the active (light generating) types such as: Light emitting diodes arrays (LED); Plasma panels (GDD); and Electroluminescent displays (ELD). Liquid crystal displays (LCD) are by far the most successful flat display on the market today. They are relatively economical to build, provide a fair contrast image with a resolution which is now comparable to the cathode ray tube. They can also provide a color image with somewhat lower resolution than monochrome. However, they have the drawback of providing a variable contrast with the angle of viewing the image. The addressing time is still relatively slow and they need complicated circuitry for TV scanning.
Magneto optics displays (MOD) are still very expensive to produce due to the fact that they require single crystals which are necessarily expensive as large crystals or need an assembly or smaller crystals in a mosaic structure. Magneto optics displays are also difficult to address and require higher signal voltages than the liquid crystal display.
Electrochromic display (ECD) are the next best choice for a flat display after liquid crystals. They have the advantage over the liquid crystal display in providing an optical density based on the absorption of light rather than on scattering or on modulation of polarized light, which eliminates the variation of contrast with the direction of viewing. They are simpler in structure than the liquid crystal display and would probably lead to more economical manufacturing. The drawback at present is their slow response time and short life.
In the category of active displays, which present the inconvenience of restricted viewing in high light environments, the light emitting diodes array (LED) is an expensive display both from the point of view of manufacturing cost and power consumption. Also the resolution is limited by the smallest size of the individual diodes. Color display is possible but with much lower resolution than the passive liquid crystal display.
Plasma panels (GDD) have proven to be competitive with the LED arrays at a much lower price. The main drawbacks are high driving voltage and high production cost.
The electroluminescent display (ELD), especially the thin film ELD, is probably the only active display which can compete with the passive liquid crystal display. This type of display has a higher resolution and is economical to manufacture. However, color display still presents a problem, as does lifetime.
The specification of U.S. Pat. No. 4,091,375 discloses a novel flat display device that is dissimilar to any one of the prior art devices described above. This device is a flat panel which comprises, in order, a composite electron injecting means composed of a metal layer capable of electron injection, a thin dielectric layer and a very thin metal layer, a semiconducting metal oxide layer, and a conductive layer. When the metal electron injection layer is negatively biased with respect to the conductive layer, the electron injection layer in the composite electron injecting means is brought to a negative potential relative to the very thin metal layer, whereupon the electrons in the vicinity of the Fermi level in the electron injection layer are injected into the semiconducting metal oxide layer and trapped at the energy levels of the structural defects in that layer so as to produce color centers for effecting information display. This flat display device has many advantages such as quick response, high resolution and contrast, reduced variation in contrast at varying angles of viewing, a capacity of providing different colors at the same addressing point, and operability at low signal voltage and current. However, this flat display device is difficult to manufacture and its production cost is high because the electron injecting means it employs has a complex structure.